I'm curious, about something... how is it you can 'look in' on her at all times?
Bottom line is that you need to find someone..another adult to talk to him...advise him..that he respects that is close to the situation. Who else is in the family that is one of her caretakers?
If he won't listen to you, I don't know how else to advise . You can't take over her legal guardianship without his signing over her parental rights to you. I'll bet he won't have any part of that.
School counselor or church member...priest...etc...also should get involved. Maybe where you sister goes to school.